For four years, environmentalists and others have tried to derail the project, saying extracting the oil from Alberta tar sands would increase global warming. President Barack Obama has thwarted it twice, and Nebraska officials objected initially that the route would jeopardize ecologically sensitive lands.

But the Nebraska route has been changed, and project supporters have won more backing by arguing it would create thousands of jobs. A recent State Department report raised no major objections to the project.